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This paper reports a single-crystal unpolarized-light FTIR study in the OH-stretching region of a suite of well-characterized Ti-rich pargasites from Lherz (French Pyrenees). All amphiboles studied have fairly constant M-site composition, with  [6] Al tot  ~0.55 atoms per formula unit (apfu),  [6] Ti ~0.45 apfu, and  [6] Fe 3+  ~0.40 apfu. SIMS and SREF data show all samples to have an O3 anion composition of OH ≈ O 2-  ≈ 1.0 apfu, with negligible F. The FTIR spectra show for all samples a broad absorption consisting of several overlapping bands; three main components can be recognized: ~3710, 3686, and 3660 cm -1 , respectively, with an asymmetric tail extending to lower frequency. Six Gaussian components can be fitted to the spectra; comparison with spectra of both synthetic and natural pargasites allows five of these components to be assigned to local configurations involving OH-O 2-  at the O3 site, thus showing that coupling with an O 2-  anion through an A-cation significantly affects band position. Infrared spectroscopy can detect the presence of O 2-  in amphiboles in chemically favorable cases, i.e., in the absence of F. Moreover, the FTIR spectra show that all octahedral configurations involving  M1 Ti 4+  or  M1 Fe 3+  M3 Fe 3+  are associated with O 2-  at both adjacent O3 sites, and that  M3 Al is locally associated with OH, confirming SRO models based on structure refinement results.

FTIR spectroscopy of Ti-rich pargasites from Lherz and the detection of O 2- at the anionic O3 site in amphiboles